Nerve impulse

When an action potential arrives at the end of the pre-synaptic axon (yellow), it causes the release of neurotransmitter molecules that open ion channels in the post-synaptic neuron (green). The combined potentials of the inputs can begin a new action potential in the post-synaptic neuron.

The action potential is the result of ions moving in and out of the cell.[2] Specifically, it involves potassium (K+) and sodium (Na+) ions. The ions are moved in and out of the cell by potassium channels, sodium channels and the sodium-potassium pump.

Axons of neurons are wrapped by several myelin sheaths, which shield the axon from extracellular fluid. There are short gaps between the myelin sheaths known as nodes of Ranvier where the axon is directly exposed to the surrounding extracellular fluid.

Faster electrical synapses are used in escape reflexes, the retina of vertebrates, and the heart. They are faster because they do not need the slow diffusion of neurotransmitters across the synaptic gap. Therefore, electrical synapses are used whenever fast response and coordination of timing are crucial.

These synapses connect the presynaptic and postsynaptic cells directly together.[3] When an action potential reaches such a synapse, the ionic currents cross the two cell membranes and enter the postsynaptic cell through pores known as connexons.[4] Thus, presynaptic action potential directly stimulates the postsynaptic cell.